Physical Science - Astronomical Phenomena Notes

Learning Objectives

  • Explain how the Greeks knew that the Earth is spherical. [S11/12PS-IVa- 38]
  • Cite examples of astronomical phenomena known before the telescope. [S11/12PS-IVa-41]
  • Explain Brahe’s contributions and how they influenced Kepler’s laws of planetary motion. [S11/12PS-IVb-44]

Overview of Astronomical Phenomena

  • Key Phenomena Studied Pre-Telescope:

    1. Motion of Stars/Constellations
    2. Phases of the Moon
    3. Eclipses
    4. Planets

  • Definition of Astronomy:

    • Science that studies positions, dimensions, and movements of celestial bodies; oldest natural science related to religious, cosmological, and astrological beliefs.

Motion of Stars and Constellations

  • Sun Motion:
    • The Sun rises in the east and sets in the west, forming the basis for the day's duration.
  • Earth’s Tilt:
    • Seasons change due to Earth’s axial tilt (~$23.5^ ext{°}$) during its orbit around the Sun.
Equinox and Solstice
  • Equinox:
    • Time when the Sun is nearest the equatorial plane, resulting in equal day/night lengths.
  • Solstice:
    • Time when the Sun is farthest from the equatorial plane, resulting in unequal day/night lengths.
Key Dates
  • Winter Solstice: December 21-22
  • Vernal Equinox: March 20-21
  • Summer Solstice: June 20-22
  • Autumnal Equinox: September 22-23
Perihelion and Aphelion
  • Aphelion:
    • Greatest distance between a planet and the Sun.
  • Perihelion:
    • Closest distance to the Sun.
  • Orbital Speeds:
    • Slowest at aphelion, fastest at perihelion.

Cultural Significance of Celestial Motion

  • Navigation and Agriculture:
    • Celestial movements influenced ancient activities like agriculture, warfare, and timekeeping.
  • Ancient Cultures:
    • Included Babylonian, Egyptian, Greek, and Mayan, each with unique astronomical records and structures for aligning with celestial events.
Ancient Contributions
  • Babylonian Astronomy:
    • Documented sun and moon movements; used for political warnings and agriculture.
  • Egyptians:
    • Built pyramids aligned to stars; used sundials and recognized a 365-day year with leap years.
  • Mayan Civilization:
    • Integrated calendars with mythology; predicted eclipses and celestial events.

Phases of the Moon

  • Understanding Moon Phases:

    • Key to calendar construction and monthly cycles.

  • Phases Overview:

    1. New Moon: Between Earth and Sun (invisible)
    2. Waxing Crescent: Small visible portion
    3. First Quarter: Half-circle visible
    4. Waxing Gibbous: More than half visible
    5. Full Moon: Fully illuminated, Earth between Sun and Moon
    6. Waning Gibbous: Decreasing illumination
    7. Last Quarter: Opposite side of First Quarter
    8. Waning Crescent: Small visible decreasing portion

Eclipses

  • Definition:
    • Occurs when one celestial body moves into the shadow of another.
Types of Eclipses
  • Solar Eclipse:
    • Moon blocks sunlight to Earth.
  • Lunar Eclipse:
    • Earth blocks sunlight from reaching the Moon.
Cultural Interpretations
  • Various cultures attributed supernatural meanings to eclipses, viewing them as omens or myths involving gods and creatures.
Shadow Terminology
  • Umbra: Dark center shadow
  • Penumbra: Lighter outer shadow
  • Antumbra: Shadow part seen at a distance

Planets

  • Five Known Planets:
    • Mercury, Venus, Mars, Jupiter, Saturn
    • Called 'wandering stars' due to their motion relative to fixed stars.
Planet Characteristics
  • Mercury: Smallest and fastest, named after the messenger god.
  • Venus: Known as